Inhibition of corrosion in cooling water systems with mixtures of gluconate salts and silicate salts

ABSTRACT

OXIDATIVE CORROSION IN COOLING WATER SYSTEMS IS SUBSTANTIALLY REDUCED BY THE ADDITION OF SMALL AMOUNTS OF A SILICATE AND A GLUCONATE. A POLYPHOSPHATE CAN ALSO BE ADDED IF DESIRED.

United States Patent M US. Cl. 212.7 7 Claims ABSTRACT OF THE DISCLOSUREOxidative corrosion in cooling water systems is substantially reduced bythe addition of small amounts of a silicate and a gluconate. Apolyphosphate can also be added if desired.

BACKGROUND OF THE INVENTION This invention relates to methods ofpreventing oxidative corrosion of metals by aqueous solutions. Inparticular, this invention relates to methods of inhibiting oxidativecorrosion in recirculating cooling water systems.

Cooling water systems are widely used in oil refineries and in chemicalplants, as well as in homes, factories and public buildings. Each dayhuge volumes of water are circulated through tremendous numbers of suchsystems. This obviously represents a large dollar volume in capitalinvestment and operating expense.

Cooling water systems may be classified generally into two types. Onetype is the once-through cooling system, where cooling water is pickedup from a convenient source, such as a river, sent once through thecooling equipment, and then discharged. Corrosion problems in suchsystems are generally minor. However, in most localities cooling wateris not sufiiciently abundant to permit the use of a once-through systemand the number of such systems is on the decrease.

The other general type of cooling water system is the recirculatingcooling water system. Recirculating systems include a cooling tower orequivalent type of equipment. Heat picked up by the water in suchsystems is passed on to the atmosphere by passing air through the heatedwater in the cooling tower or equivalent equipment. However, during thecourse of such contact with the air, a substantial amount of airdissolves in the cooling water and is ciirculated throughout the coolingsystem. The oxygen dissolved in the water dilfuses to the water-metalinterface and will produce corrosion in the heat exchangers and on themetal pipes and vessels in the cooling system. Admiralty metal, copper,and steel, particularly carbon steels, are the most commonly usedmaterials in such systems, and unfortunately such materials areparticularly prone to oxidative attack.

The prior art has recognized this problem and has attempted to inhibitthis oxidative corrosion in water cooling systems by introducing variousinorganic inhibitor systems which produce thin metal oxide films on themetal surfaces of the cooling systems so as to retard or hopefullyprevent the diffusion of oxygen to the metal surfaces. Substances whichhave achieved wide acceptance in the art for this purpose includechromate and phosphate salts. Silicate salts have also been used forthis purpose. Unfortunately, these substances have serious drawbackswhen used as corrosion inhibitors.

Chromates under certain conditions can give rise to acceleratedcorrosion. For example, chromates can promote pitting when introduced inlow concentrations. This pitting attack may be quite serious and mayresult in perforation, particularly in areas of breaks or discon-3,711,246 Patented Jan. 16, 12%73 tinuities in the film produced by thechromate inhibitor. Since setting up virtually perfect thin film inlarge scale equipment with high flow rates is tricky to say the least,it is safe to say that effective inhibition will be most unpredictablefrom unit to unit, and even from day to day in the same unit.

:A further and most serious drawback in the use of chromates asinhibitors arises from the fact that chromates are pollutants. Chromateshave toxic properties and their presence in streams and rivers is comingunder everstricter control in new anti-pollution laws. Thus, in order tobe able to circulate used cooling water with an environmental sewagesystem, it would be necessary for the cooling system operator to installadequate purification equipment to remove the chromate prior to waterdisposal. This procedure adds substantially to plant investment andoperating costs. As a practical matter, it is very difiicult andprohibitively expensive to remove chromate to an adequately low level,with the result that chromate is rapidly falling into disuse as acorrosion inhibitor.

Polyphosphates have also been used as corrosion inhibitors. Thesesubstances have the further advantage of acting as sequestering agentsfor calcium and magnesium ions which are frequently present in coolingwater. However, it is known that polyphosphates are quite corrosive inconcentrated solutions and that under certain conditions when used inhigh concentrations they sufier from conversion to orthophosphates withthe resulting formation of sludge or scale which can promote seriouscorrosion. In addition, polyphosphates are also stream pollutants whendischarged into a sewage system, although the acceptable concentrationof phosphates is considerably higher than the acceptable concentrationof chromates.

Alkali metal silicates have also been suggested as corrosion inhibitors.A problem with alkali metal silicates is that they promote the formationof scale in pipes and cooling equipment, especially when calcium ormagnesium is present in the cooling water, thereby promoting corrosionand fouling.

There exists a need for a new and effective corrosion inhibitorcombination which will effectively inhibit corrosion of metal surfacesin cooling water systems While at the same time does not result inexcessive concentrations of pollutants which cannot be discharged intoenvironmental sewage ssytems. It has previously been proposed to inhibitthe corrosion of metal surfaces in cooling water systems by adding tothe water an alkali metal or ammonium gluconate. Sodium gluconate hasbeen particularly suggested for this purpose. The gluconate salts arenot toxic in the concentrations utilized and do not pose pollutionproblems if discharged into environmental waste water systems.Gluconates are suitable for preventing corrosion of soft waters havinglow concentrations of calcium or magnesium, although the minimumconcentration required is generally somewhat higher than in the case ofpolyphosphates. The gluconates are not quite effective in inhibitingcorrosion in hard waters having substantial concentrations of calciumand magnesium. Hence, the art is seeking new corrosion inhibitorcombinations for cooling water systems which will effectively inhibitcorrosion and at the same time will not cause stream pollution whenwaste water is discharged from the system.

SUMMARY OF THE INVENTION It has now been found that small amounts of aninhibitor combination comprising a water-soluble inorganic silicate saltand a water-soluble inorganic gluconate salt is effective as anoxidative corrosion inhibitor in recirculating cooling water systems. Asmall amount of a water-soluble polyphosphate salt can also be includedin the inhibitor combination. Cooling water systems can be inhibitedagainst corrosion simply by adding the silicate salt, the gluconatesalt, and the polyphosphate salt also where desired, to the coolingwater in elfective corrosion inhibiting amounts.

DESCRIPTION OF THE PREFERRED EMBODIMENT The Water-soluble silicate saltsfor use in the present invention are the water-soluble alkali metalsilicates. These may be represented generically by the formula Na O-xSiO-yH O, where x is in the range of about 1 to about 3.5. Commercialsodium silicate solutions in which the mole ratio of silica to soda isabout 3.3 may be used to advantage. However, more alkaline solutionshaving an SiO :Na O mole ratio as low as about 1:1, or less alkalinesolutions having an SiO :Na O mole ratio up to about 3.5:1 can also beused.

The preferred inorganic water-soluble gluconate salts are the alkalimetal gluconates and ammonium gluconate. The term, inorganic gluconate,as used herein denotes a gluconate salt having an inorganic cation, suchas an alkali metal or ammonium. The suitable alkali metal gluconatesinclude lithium gluconate, sodium gluconate and potassium gluconate.Sodium gluconate is especially preferred because it gives an effectiveinhibition and is an article of commerce which is readily available atrelatively low cost.

Preferred polyphosphates for use in the present invention are thewater-soluble inorganic hexametaphosphates, and particularly the alkalimetal and ammonium hexametaphosphates. Sodium hexametaphosphate is apreferred salt of this type. Sodium hexametaphosphate is a readilyavailable article of commerce which is used as the chelating agent inaqueous systems containing calcium and magnesium ions. Polyphosphatesare primarily useful in cooling Water systems using hard water, sincethey prevent or minimize calcium precipitation. In addition, theyimprove the corrosion inhibiting properties of the silicate-gluconatecombination at concentrations low enough to be acceptable from thestandpoint of stream pollution.

The concentration of alkali metal silicate in the cooling wateraccording to this invention is in the range of about 2 to about 200p.p.m., and preferably about 10 to about 80 p.p.m. The water-solublegluconate salt is present in a concentration of about 2 to about 1000p.p.m., and preferably about 20 to about 90 p.p.m. The watersolublepolyphosphate is present in a concentration of about to about 100 p.p.m.

The silicate, gluconate, and polyphosphate where used, can be added aseither solids or as aqueous solutions to the cooling water system inamounts which will give the above-specified concentrations. The alkalimetal silicate, for example, may be a commercially available sodiumsilicate (water glass) solution containing approximately 3.3 moles ofsilica per mole of soda; other silicate solutions or alkali metalsilicates in solid form may be used instead. The gluconate andpolyphosphate salts will most often be added as solids, although againaqueous solutions are acceptable.

It is believed that the contact of the cooling water containingdissolved silicate, gluconate, and (optionally) phosphate salts in thedesired concentration range with metal surfaces of the cooling watersystem results in the formation of a thin protective film on these metalsurfaces. This film serves to inhibit the diffusion of oxygen from thewater phase to the metal surfaces, thereby substantially lowering thecorrosion rate of the metal.

In some applications, a single treatment with the inhibitor combinationwill be sufficient to adequately protect a cooling water system up totwo or three weeks. However, in cases where there is unusually turbulentflow or a vessel configuration which makes it difficult to preserve filmintegrity on the metal surfaces, or in instances where inhibitor andwater losses are excessive, it may be necessary to repeat the additionof inhibitors on occasion or alternatively to maintain a continuousinhibitor concentration in the desired range by constant addition ofinhibitors in order to preserve the protective film. Generally, a higherconcentrationof inhibitor is required on startup of a cooling watersystem than is required thereafter. Thus, it is generally desirable toadd enough inhibitor to establish a total inhibitor concentration of atleast about 50 p.p.m. when a system is being started up. This will causea protective film to form on the metal surfaces in the system. Once thisfilm is formed, it can frequently be maintained at lower inhibitorconcentrations, in many cases as low as about 25 totalinhibitorconcentration.

The present invention will be described more fully with reference to thefollowing example.

Example This example describes the efiicacy of a mixture of sodiumsilicate and sodium gluconate as an inhibitor of oxidative corrosion incarbon steel exposed to water saturated with dissolved oxygen.

The test procedure involves placing a small specimen of known weight of1020 carbon steel (1" x 4" x A") in tap water through which air isconstantly being bubbled. The concentration of dissolved oxygen willthus be kept at a very high level and will duplicate a long period ofexposure of the-metal in a cooling water system environment. A secondspecimen of 1020 carbon steel of known weight is placed in water whichis also saturated with air by means of a bubbler. A desired quantity ofthe corrosion inhibitor is dissolved in this Water. The temperature ofboth the blank and the testsolutions is maintained at 120 F.

The specimens before their introduction into the solutions are abradedthrough 4-0 emery paper, degreased in benzene, pickled in dilutesulfuric acid and washed in distilled water.

After six days the samples are removed, cleaned with a soft brush,washed with Water and then acetone, and are weighed after drying. Theamount of corroded metal is determined by weight loss by weighing beforeand after the test. The corrosion rate is calculated in mils per year, amil being .001 inch. The effectiveness of an inhibitor to reducecorrosion is expressed as percentage inhibitor efiiciency where E equalsinhibitor elficiency, I is the corrosion rate without inhibitor, and Iis the corrosion rate with inhibitor.

The inhibitor salts in this example were sodium silicate and sodiumgluconate. The sodium silicate was commercial sodium silicate having anNa O:SiO mole ratio of 1:1. The sodium gluconate'wascommercial anhydroussodium gluconate. For comparison purposes a blank run (no inhibitor) andsamples using sodium silicate alone and sodium gluconate alone were alsotested by the same procedure.

Results of representative experiments using the above procedure andinhibitor combination are summarized in Table I below.

Good corrosion inhibition is also obtained in solutions containing asilicate, a gluconate, and a polyphosphate. For example, a solutioncontaining 50 p.p.m. of sodium silicate, 25 p.p.m. of sodium gluconate,and 25 p.p.m. of sodium hexametaphosphate, when tested under theconditions of the above example, gave a corrosion rate of 5.7 mils peryear and an inhibitor efiiciency of 70%. This inhibitor efliciency isnearly as great as the inhibitor efliciency obtained with 50 p.p.m. ofsodium silicate and 50 p.p.m. of sodium gluconate, in spite of the lowercombined concentrations of silicate and gluconate. Sodiumhexametaphosphate appears to make some contribution to the corrosioninhibiting properties of the solution in addition to serving as asequestering agent for calcium ions.

What is claimed is:

1. A method of inhibiting oxidative corrosion in a circulating coolingwater system in which the cooling water contains substantial amounts ofdissolved oxygen, said method comprising adding an alkali metal silicateand an alkali metal or ammonium gluconate salt to said water in amountsgiving a corrosion inhibited aqueous solution containing about 2 to 200parts per million of said alkali metal silicate and about 2 to about1000 parts per million of said gluconate.

2. A method according to claim 1 in which an alkali metal or ammoniumpolyphosphate salt is also added to said water in an amount giving aconcentration of to about 100 parts per million of said polyphosphate insaid corrosion inhibited aqueous solution.

3. A method according to claim 2 in which said polyphosphate salt is analkali metal hexametaphosphate.

4. A method according to claim 1 in which said silicate and saidgluconate are added in amounts giving a corrosion inhibited aqueoussolution containing about to about 80 parts per million of said alkalimetal silicate and about to about 90 parts per million of saidgluconate.

5. A method according to claim 4 in which an alkali metal or ammoniumpolyphosphate salt is also added to said water in an amount giving aconcentration of 0 to about 100 parts per million of said polyphosphatein said corrosion inhibited aqueous solution.

6. The method according to claim 1 in which said silicate and saidgluconate are added in amounts of about 10 to about parts by weight ofsaid silicate and about 20 to about parts by weight of said gluconate,based on parts of combined weight of said silicate and said gluconate,

7. A method of inhibiting oxidative corrosion in a circulating coolingwater system which comprises forming a corrosion inhibited aqueoussolution consisting essentially of water having oxygen dissolvedtherein, about 2 to 200 parts per million of an alkali metal silicate,about 2 to 1000 parts per million of a water-soluble alkali metal orammonium gluconate salt, and about 0 to 100 parts per million of awater-soluble alkali metal or ammonium polyphosphate salt, andcirculating said solution through said system.

References Cited UNITED STATES PATENTS 3,235,404 2/1966 Mickelson et al.204-38 B 3,110,684 11/1963 Miller 21-2.7 2,481,977 9/1949 Cinamon252-387 2,909,490 10/1959 Metziger 252-387 3,580,934 5/1971 Murray et al252-387 LEON D. ROSDO'L, Primary Examiner I. GLUCK, Assistant ExaminerU.S. Cl. X.'R.

